The present invention relates to a cordless key telephone system.
Cordless key telephone systems have been in use today replacing wired key telephone systems as they considerably reduce the amount of cables and wires laid on office floors the arrangement of which must be altered to meet current needs of working environment. The system includes a main controller which is coupled to the public or private switched telephone network via subscriber loops to receive incoming calls and originate outgoing calls. The floor space which the system covers is divided into several service areas. One or more access units are located in each service area and coupled to the main controller to exchange control signals with it. The system has only one control channel for exchanging control signals between the access units and cordless stations and several speech channels for transmitting ringing signals and speech signals. In response to an incoming call, the main controller selects one access unit for each service area and sends an alert signal to it over the control channel. Each selected access unit then broadcasts a recurrent sequence of alert signals respectively containing the identifiers of all standby cordless stations to elicit a response therefrom. Each standby cordless station responds to the alert signal addressed to it by returning an acknowledgment signal containing it in the own station identifier as well as the system identifier.
However, since the boundaries between adjacent service areas are not sharply defined, the adjacent areas tend to partially overlap with each other, creating an area in which collision is likely to occur between control signals transmitted from the access units of the adjacent area when an incoming call is received.
Interference between control channel and speech channels is another problem. If the control channel is severely affected by noise from adjacent channels due to intermodulation distortion, an undesirable situation can occur.